The invention relates to a process for the preparation of copolymers of carbon monoxide and one or more ethylenically unsaturated compounds.
Linear copolymers of carbon monoxide with one or more ethylenically unsaturated compounds, in which copolymers the units originating from carbon monoxide on the one hand and the units originating from the ethylenically unsaturated compound(s) used on the other hand occur in a substantially alternating arrangement, can be prepared by contacting the monomers at an elevated temperature and pressure, in the presence of a diluent in which the copolymers are insoluble or virtually insoluble, with a suitable catalyst. The preparation of the copolymers can, in principle, be carried out by employing either a batchwise or continuous process.
Batchwise preparation of the copolymers is generally carried out by introducing the suitable catalyst into a reactor containing diluent and monomers and which is at the desired temperature and pressure. As polymerization proceeds, the pressure drops, the concentration of the copolymers in the diluent increases and the viscosity of the suspension rises.
The reaction is discontinued when the viscosity becomes very high because of severe agglomeration of copolymer particles and continuing further could create difficulties in view of poor heat transfer in the suspension and stirring problems.
In principle, the only parameter which remains constant in batchwise copolymer preparation is the temperature. A variant of batchwise copolymer preparation is semi-batchwise preparation in which besides the temperature the pressure is also kept constant by adding monomers to the reactor during the polymerization.
In continuous copolymer preparation, a diluent, monomers and catalyst are continuously added to a reactor containing monomers and a certain volume of the diluent and which is at the desired temperature and pressure and a copolymer suspension is continuously withdrawn from it. During the continuous copolymer preparation, the temperature, the pressure and the liquid volume in the reactor are kept substantially constant. After a start-up period in which the copolymer concentration in the suspension increases to the desired value, a stationary state is reached which is characterized, inter alia, by the suspension which is withdrawn from the reactor having a substantially constant copolymer content and the copolymers contained therein having substantially constant properties.
One of the important properties of the copolymers in question is the bulk density. This plays an important role both in the preparation, treatment, storage, transport and processing of the copolymers. As far as copolymer preparation is concerned, the rule of thumb is that the maximum permissible suspension concentration, expressed in (kg copolymer/kg suspension).times.100, is about one tenth of the bulk density expressed in kg/m.sup.3. This means that in the preparation of a copolymer with a bulk density of 100 kg/m.sup.3, the maximum suspension concentration is about 10%, whereas in the preparation of a copolymer with a bulk density of 200 kg/m.sup.3, the maximum suspension concentration is about 20%. Therefore, a doubling of bulk density offers the possibility of preparing about twice the quantity of copolymer in the same reactor volume. Regarding copolymer treatment, such as filtering, washing and drying, is concerned, the quantity of adhering liquid is largely determined by the copolymer bulk density. Thus it has been found, for example, that a copolymer with a bulk density of 100 kg/m.sup.3 binds approximately 5 kg diluent or washing liquid per kg, whereas to a copolymer with a bulk density of 200 kg/m.sup.3 considerably less liquid is bound. This is naturally of great importance because of the quantity of liquid that must be used in washing the copolymers and must be removed when the copolymers are dried. Regarding transport and storage, the rule is that the higher the copolymers' bulk density, the more attractive the flow behavior they will have and the less space they will occupy. As regards the processing of the copolymers into shaped objects, it is often the case that copolymers with low bulk density cause problems in the processing apparatus. For copolymers with low bulk density of below 250 kg/m.sup.3 a compacting treatment for example by extrusion, in order to make them suitable for further processing in the usual equipment for that purpose could become a problem. The higher the copolymers' bulk density, the easier the prior treatment, particularly extrusion, of the material.
It will be clear from the above that the main interest is in a process whereby a relatively high suspension concentration is achieved and whereby copolymers having a high bulk density are produced. However, a complication is formed by the previously mentioned agglomeration of copolymer particles, which may occur, before the suspension concentration has reached a value required for obtaining copolymers in an adequate yield.
An investigation of this phenomenon has revealed that the moment of severe agglomeration can be somewhat postponed by increasing the amount of catalyst used. However, when operating the process on a commercial scale, for economic reasons, the use of large quantities of catalyst is undesirable.